1. Field of the Invention
The present invention relates generally to room air cooling and dehumidification, and more particularly, to a liquid desiccant air conditioner including a liquid desiccant dehumidifier which is energy efficient, corrosion resistant, and capable of operation with low energy usage.
2. Description of the Prior Art
Typical air conditioning units operate on a vapor compression cycle. Over recent years, the phase out of CFC based air conditioning units has been dictated by environmental concerns. One alternative to vapor compression units, is the absorption system. The basic elements include an evaporator, condenser, absorber, pump, heat exchanger, throttle valve and regenerator. In the absorption cycle, an "absorbent" is used to absorb the refrigerant in the vaporized state after leaving the evaporator. The vaporized refrigerant is converted back into the liquid phase in the absorber. Heat released in the absorption process is rejected to cooling water passed through the absorber. A solution of absorbent and refrigerant is pumped to a regenerator, where heat is added and the more volatile refrigerant is separated from the absorbent through distillation. The refrigerant is then communicated to the condenser, expansion valve and evaporator in a conventional manner. A heat exchanger may be used for heat recovery between the absorbent returned to the absorber and the absorbent refrigerant solution delivered to the regenerator.
Absorption systems currently represent only a small percentage of commercial refrigeration systems because they are generally bulky and inefficient. However, with concerns over CFCs and ever increasing energy costs, the absorption unit has potential to provide efficient cooling by taking advantage of waste heat. This may be provided by combining such an absorption unit with a liquid desiccant dehumidifier.
It is known in the art to dehumidify ambient air using liquid desiccant systems. These devices typically utilize hygroscopic liquids such as lithium bromide (LiBr), lithium chloride (LiCI) or calcium chloride (CaC12) as the desiccant solution. In a desiccant system, the desiccant solution absorbs moisture from ambient air exposed to the solution. As the desiccant solution continues to absorb moisture, it becomes dilute and must be regenerated. In the regeneration process, the desiccant solution is heated to evaporate the excess moisture or the desiccant solution is brought into contact with a hot gas to desorb the excess moisture. In some expedients, air regenerators are used to regenerate the desiccant. These arrangements have relatively high operating costs as energy is required to provide a source of heat and to generate a suitable flow of air. In others, boiler-type regenerators are employed. However, boiler embodiments are expensive, as the corrosive nature of liquid desiccant solutions necessitates the use of costly corrosion resistant metals.
A liquid desiccant dehumidification system in which a liquid desiccant is regenerated with a boiler is described in U.S. Pat. No. 4,939,906 ("the '906 Patent"). The '906 Patent discloses a gas-fired desiccant boiler and a combined desiccant regenerator/interchange heat exchanger in which the combined regenerator/heat exchanger utilizes steam produced from the boiler to provide heat for partial regeneration. The desiccant boiler has a liquid/vapor separator chamber and then-nosyphon recirculation to reduce scale and corrosion of the boiler. Specifically, the overall system is shown in FIG. 1, wherein outdoor air is drawn into the system through an inlet duct 22, and is evaporatively cooled by a water spray 24. The cooled air is directed to a desiccant conditioner 26 to which return air is also directed through a duct 30. In the desiccant conditioner 26, the return air is contacted with a liquid desiccant solution from a sprayer 28. The desiccant liquid is disclosed as lithium calcium chloride.
This dehumidified air is then supplied to the space to be dehumidified, or it can be sensibly cooled through an evaporative cooler 32. The desiccant dehumidifies the air stream, and in the process its moisture-absorbing capability is reduced. This capability is regenerated by passing a portion of the dilute desiccant from the conditioner 26 to a first interchange heat exchanger 44 wherein the temperature of the desiccant is raised. The weakened desiccant is partially concentrated in an air-desiccant regenerator 46 in which heated air from a regeneration air heater 48 contacts the liquid desiccant. This desiccant is pumped through a second interchange heat exchanger 52 and thereafter to a desiccant boiler 56 in which regeneration of the desiccant is completed. The water vapor generated in the desiccant boiler 56 raises the temperature of the air passing through the regeneration air preheater 48. The interchange heat exchangers 44, 52 reduce the temperature of the regenerated desiccant as it returns along the pipe 60 to the conditioner 26.
The boiler 56 is depicted in FIG. 2 and operates on natural circulation, with the density of the fluid (part liquid, part vapor) in the "fired" tubes 70 being less than the density of the liquid in the outer "unfired" tube 74. A porous ceramic burner 80 facilitates combustion to provide a heat source, and hot combustion gases are blown through a combustion chamber formed by a housing 88 enclosing the fired tubes 70 so as to effect flow across fins 90 of the fired tubes 70. Weak desiccant is pumped into the fired tubes 70 through a manifold 94 which causes water in the desiccant to be vaporized. Accordingly, a density differential is created between the fluid in the fired tubes 70 and the unfired tubes 74 connected between the manifold 94 and a liquid/vapor separator 98 outside the combustion chamber housing 88. This density differential induces a natural flow of desiccant solution up the fired tubes 70 and down the unfired tubes 72. In this mainer, the natural circulation of desiccant keeps the inside walls of the fired tubes 70 coated with desiccant to thereby reduce or prevent "hot spots" from forming on the inside of the fired tubes 70 to reduce corrosion and scale build up in the fired tubes 70.
The liquid vapor separator 98 at the top of the boiler 56 separates water vapor from the concentrated liquid desiccant. A portion of the concentrated desiccant is withdrawn from the bottom of the liquid/vapor separator 98 and is returned to the desiccant conditioner 26. Water vapor flowing out of the top of the liquid/vapor separator 98 is subsequently condensed to heat air for use in an earlier regeneration step shown in FIGS. 3 and 4.
The combined regenerator/interchange heat exchanger, depicted in FIGS. 3 and 4, comprises two (2) interchange heat exchangers 44, 52, the desiccant regenerator 46 and the regeneration air heater 48. The combined desiccant regenerator/interchange heat exchanger is identified by the reference numeral 102, and is constructed by alternately stacking two (2) different corrugated plates (see FIG. 4) to define alternating flow channels. Water vapor or steam from the desiccant boiler 56 is introduced near the top of the regenerator/exchanger 102 in alternate channels (plate A). This water vapor is condensed, thereby transferring heat to the air and weak desiccant entering adjacent channels near the top of the regenerator/heat exchanger 102 (plate B). The upper portion of each plate corresponds to the desiccant regenerator 46 and regeneration air heater 48. As the water vapor condenses, the weak desiccant and air mixture is heated and the desiccant is partially regenerated. Warm air and moisture are exhausted by fan 106 to the outdoors. An entrainer 108 is provided to prevent desiccant from escaping the combined regenerator/exchanger 102. The partially regenerated desiccant flows into the middle of a channel plate B, and is further heated by the hot concentrated desiccant removed from the liquid/ vapor separator 98. Hot concentrated desiccant from the boiler 56 is introduced at the middle of plate A while the partially regenerated desiccant is removed from the middle of plate B. The partially regenerated desiccant is then pumped to the desiccant boiler 56. Diluted desiccant from the regenerator/heat exchanger 102 is introduced at the bottom of the plate A and is heated by the hot desiccant from the boiler 56. The heated dilute desiccant from the regenerator/heat exchanger 102 is then removed from the center of plate B and pumped to the top of plate B.
The apparatus shown and described in the '906 patent exhibits several disadvantages. The regeneration process described therein requires the flow of hot air through the system in order to operate. This necessitates the use of additional components such as fans, air preheaters, and liquid/vapor separators, which add system complexity. Furthermore, the multiple stacked plate interchange heat exchanger configuration is complex and takes up a relatively large amount of space.
The present invention provides an improved air cooling system comprising an absorption air conditioner operating in conjunction with a liquid desiccant dehumidifier.